This invention relates to an article made of a thermoplastic foam, in particular an energy absorption unit suitable for use in absorbing energy transferred during an impact between two bodies and especially an automobile energy absorbing unit, for example a vehicle bumper.
An energy-absorbing unit (EAU) typically forms part of or is attached to an article to provide a means of reducing damage to the article during impact with a body and also often to reduce damage to the body. When employed as a vehicle bumper, an EAU suitably acts to reduce damage both to the vehicle and also to the body with which impact is made.
In recent years, plastic automobile EAUs have been employed in conjunction with conventional metal automobile bumpers to absorb low energy (lower speed) impacts. Energy absorption units employed as vehicle bumpers typically have a bumper fascia moulding on one side, usually composed of a core material of a plastic foam and a surface material of a synthetic resin enclosing the foam core and are usually mounted forward of a stiff cross beam. The foam core material is an important component part which affects the performance of an automobile EAU, and is generally required to have excellent energy absorbing properties and may require dimensional recovery. Further, to decrease the weight of an automobile, the EAU core material should be of low density.
Plastic materials which have been proposed for the foam core material include polyurethane, polyethylene bead, polystyrene and polypropylene bead foams. A conventional automobile EAU core of foamed olefinic polymer is typically prepared by feeding expandable beads of a polypropylene-type resin in a mould capable of enclosing the particles but allowing escape of gases therefrom, and heating the expandable beads at a temperature at which the beads expand and soften and melt-adhere to each other into a mass, whereby a foamed molded article expanded to the same shape and size as the cavity of the mould is formed. An automobile EAU core prepared from beads of a polypropylene type resin foam suitably has a foam density from 16 kg/m3 to 160 kg/m3 to achieve an average compressive stress of about 50 kPa to 1000 kPa at 25% compression
Vehicle bumpers which have a number of sections of differing absorption characteristics have been known for many years. For example DE 3224979A, published in 1983, discloses a bumper having three sections of differing elasticity. EP 947727A discloses an energy absorbing foam having two parallel layers with different energy absorbing characteristics. The system may be used in bumpers and reference is made to polypropylene, polystyrene and polyurethane foams.
However, current vehicle bumper systems are not specifically designed to take account of pedestrian impact in combination with the other types of impact in which the vehicle may be involved. For example, vehicle bumpers may be produced from steamchest moulded expanded polypropylene foam in beaded form which perform adequately in certain types of impact but which may be too stiff or too soft in relation to other types of impact and so do not provide optimum performance across the full range of different types of impact.
There is a continuous need for improvement in the safety of pedestrians and other road users by reducing the incidence of death and the severity of injury due to collisions with automobiles. Whilst aiming to improve pedestrian safety, it is also desirable to provide improved protection for the vehicle against low speed impacts and also reducing and desirably minimizing damage to the structure of the vehicle by way of protecting the vehicle chassis or other structural parts, so-called insurance impact protection
We have now found that an energy absorbing unit having a combination of energy absorbing characteristics which provide excellent impact protection for pedestrians, especially knee and lower leg impact, in combination with low speed impact protection and xe2x80x9cinsurancexe2x80x9d impact protection may be provided by a unit formed from a thermoplastic foam material having a plurality of zones with different energy absorbing characteristics arranged in a particular manner.